This invention relates to a fluid permeable, fibrous electric heating element for example for a heat transfer system and to said system, and includes a heat transfer system for controlling the temperature of a mould, a calender, or an extruder.
The invention also provides a process for manufacturing a fluid permeable, fibrous electric heating element comprising an electrically conductive selected material, the process comprising, the steps of, forming a precursor comprising carbon fibres, coating the fibres of the precursor with the selected material by a vapor deposition process, and then heating the coated precursor in an oxidizing environment at a temperature between 300.degree. C. and 1400.degree. C. such as to remove the fibrous carbon precursor by oxidation thereof and leave a fibrous body comprising hollow tubular fibers comprising the selected material, the tubular fibers defining a voidage between adjacent said tubular fibers in the body, and the voidage providing a path for a fluid to be heated, and heating the coated precursor at a temperature between 650.degree. C. and 1400.degree. C. to modify the structure of the coating and to lower the electrical resistivity of the coating to a desired value, the two heating steps being performed in either order or as a single step.
Preferably, the coating is produced from a plasma formed by the ionisation in an electrical field, such as a radio frequency field, of a gas or gases containing chemical elements of the selected material to be deposited.
The invention also provides a fluid permeable, fibrous electric heating element made by the process of the invention. A desired electrical resistance of the heating element may be obtained by an appropriate heat treatment which might be the oxidising step of said process, or a prior or a subsequent heat treatment.
The heating element made by the process of the invention comprises a fibrous body comprising electrically conductive tubular fibers comprising the selected material, the tubular fibers defining a voidage between adjacent said tubular fibers in the body, and the voidage providing a path for a fluid to be heated. The tubular fibers may comprise a silicon-containing material, for example silicon, silicon carbide, or silicon nitride, or silicon and carbon together with silicon carbide, or silicon together with silicon carbide, or silicon together with silicon nitride.
The heating element of the invention may be used for example in a heat transfer system comprising a circuit adapted to contain a fluid, a heating means for heating the fluid in the circuit, and a heat transfer surface portion, the heating means comprising a fluid permeable, porous electric heating element of the invention in series with at least a part of the circuit. The heat transfer surface portion may comprise a portion of a device for heating a material such for example as, a plastics material, a metal, paper, a textile, or a chemical substance.
Examples of porous electric heating elements are described in British patent specification Nos. 1466240 (U.S. Pat. No. 3,943,330), 1503644, and 1600253 (U.S. Pat. No. 4,257,157), these Specifications and Patents being incorporated by reference herein.
It is frequently necessary to heat a fluid, either a gas or a liquid, electrically and to utilise a closed loop fluid circulating system to transfer this heat to other fluids or plant in a heat exchange system. A particular example is the use of a heated liquid (e.g. oil or water) for the purpose of controlling the temperature of moulds, dies, extruders and calenders used in the plastics industry. In existing designs of such temperature control units immersion electric elements of the metal sheathed type have been used, the heat generated in these elements being transferred by conduction through the metal sheath wall to the surrounding liquid which flows past the element. There is a limit to the heat transfer coefficient which can be used with this type of element if oil breakdown or element burnout is to be avoided, e.g. 1-20 watts cm.sup.2 of the element surface area. This factor exerts a consequent effect on the metal sheath size, and the size and weight of the heating system especially at high power ratings.
Furthermore, the response of such units to a requirement for a change in the rate of heat transfer to the fluid is comparatively slow due to the appreciable heat capacity of the heating element itself, to the need to limit the element centre temperature to avoid melting the electrical conductor, and to the need to avoid high surface temperatures of the element sheath. This slow response can have a controlling influence on production cycle times when such temperature control units are used to heat moulds and dies in a production run of identical components.
The invention largely overcomes these limitations by removing the sheathed metal element and replacing it with a fluid permeable fibrous electric heating element, and circulating the fluid through the body of the permeable heating element instead of over the external surface only. Power densities exceeding 1 kW/cm.sup.3 of heating element material can then be attained, and this leads not only to a reduction in size and weight of a heating unit for a given duty but also a decrease in the response time such that the system can react virtually instantaneously to a demand for a large step increase in the heat generated and transferred to the circulating fluid.
A suitable permeable heating element might comprise silicon, silicon carbide, or a mixture of silicon and silicon carbide, or silicon, carbon and silicon carbide, of a voidage of 50-98% and a bulk density of 50-750 Kg/m.sup.3, in which the individual heating elements consist of a matrix of fine or tubular fibres of a diameter in the range 5-300 microns, the space between the fibers providing the voidage. A thermal barrier/dispenser may be used in conjunction with the heating element to produce a uniform fluid flow (See U.K. Patent No. 1 466 240), especially where there is a hydrostatic pressure head variation over the heating element entry surface.
It may be desirable to maintain a heat generation rate and a fluid flow rate through the voidage in the heating element which restricts the fluid temperature rise across the heating element to not more than about 50.degree. C., but this is not essential and temperature rises of 2.degree.-300.degree. C. or more are possible with a heating element wall thickness of 2-15 mm for an annular heating element. The heating element may have a predetermined electrical resistivity at a particular temperature, and a particular temperature coefficient of resistance, and may be suitable for mains voltage operation without the need for a transformer.
The system components include a circulating pump, a heating element assembly, valves and pipework with the oil passing in series through the element and the mould, die, extruder, or calender to be heated together with a control system. Such units have been devised and operated at heating element power ratings in the range 1-30 kW for the aforementioned heating element geometry and with a response time of a few seconds. Very compact units can be designed and for the high power units, the small size which is attainable by the use of a high heating element power density rating reduces the cost of construction considerably compared with conventional heat transfer systems.